Effective immune responses to pathogens and vaccines critically depends on the formation of germinal centers (GC) to form high affinity memory B cells and plasma cells. Despite the importance of GCs in T cell dependent immune responses, fundamental aspects of GC dynamics remain unresolved. There is a long delay after immunization, typically 5-8 days, before expansion of isotype-switched antigen-specific B cells within the follicle is evident and the formation of histologically distinct GC zones becomes discernable. The reason for this delay remains unclear. It is thought that engagement with Ag specific T helper cells (Th) at the T/B border instructs a subset of recently activated antigen specific B cells to return to the follicle and immediately proliferate, forming a germinal center. T/B collaboration at this point in the immune response appears to be dependent upon CD40 ligation. However, because the predicted immediate intra-follicular expansion of GC B cells is not typically evident, an alternative theory has emerged in which B cells that have returned to the follicle interior lie in wait for the arrival of follicular helper T cells (Tfh), proliferating at a substantially later timepoint. Here we propose to investigate the timing and location of B cell contacts with T helper cells (Th) subsets and to define the cytokine secretion profiles that lead to the initiation of the GC transcriptional program or promote the unique zonal segregation found in mature GCs. Aim 1. Hapten specific B cells will be followed for the precise timing and location of their initial intra-follicular proliferation, isotype switch, and expression of GC- associated transcription factors. We will determine whether the onset of the GC transcriptional program in B cells is coincident with either 1) the arrival of Th to the follicle interior, 2) interaction with adjacent, specific Th at the T/B border, or 3) altered IL secretion patterns at either of these locations. To define which cytokines are secreted locally, and hence correlate with the promotion of GCs, carrier specific T cells at the T/B border and each of the GC subdomains will be assessed at multiple time points post immunization for their expression of a wide variety of cytokines. Aim 2. Contact of hapten specific B cells with carrier specific T cells will be imaged by time resolved intravital multphoton microscopy and their movement tracked with post-acquisition image analysis. We will visualize these cellular contacts and the subsequent migratory fates they promote at different stages in GC development and at the distinct key locations suggested by the results of Aim 1. Aim 3. CD40/CD40L binding will be inhibited in vivo to establish the functional consequences of B cell contacts reliant on this molecular interaction. The movement of GC B cells will be tracked by time resolved intravital multiphoton microscopy after inhibition of CD40/CD40L binding to assess its role in the establishment of migration patterns that sustain GC dynamics. The proposed experiments will be an important step forward for future studies in germinal center development as well as vaccine design.